Having done a little editing in my time, I can say that it really does take an extremely large number of people to find all the mistakes in a printing of any significant size. I could look at a page in the final printing that I know I reviewed more than once and would find errors I had missed.
In the end it is about time, money and resources. In a perfect world the product would be done in plenty of time to spend a long time proofreading by lots and lots of people who had not worked on the product and would therefore be less likely to overlook mistakes because their minds know how something is supposed to read.
In reality things usually get pushed towards the latter end of a deadline, there is limited time and people to do the proofing such that people are often proofing work they have already seen before, and therefore may miss mistakes that a fresh set of eyes might catch. There is also pressure to get the product out the door without driving up costs. Finally in a product like this, increasing the number of outside people proofing the final product (say by asking a bunch of EN Worlders to take a look at it) increases the chance of something leaking out onto the internet, and good luck tracking down which internet-savvy person broke the NDA.
